A polyimide film is widely used in electronic device application because it has excellent thermal properties and electric properties.
Conventionally, a polyimide film may be produced by preparing a self-supporting film of a polyimide precursor solution, and then heating the film to effect imidization. The self-supporting film is prepared by flow-casting and applying a solution of a polyimide precursor on a support such as a stainless substrate and a stainless belt, and then heating it sufficiently to make it self-supporting, which means a stage before a common curing process; specifically, heating it at 100 to 180° C. for about 5 to 60 min.
When a support has a surface flaw, however, it is transferred to a self-supporting film, to form a flaw in the film surface. On the other hand, the opposite side of the film, which is not in contact with the support, tends to have a relatively larger surface roughness. According to the above conventional method, a flawed self-supporting film provides a flawed polyimide film, and a self-supporting film having large surface roughness provides a polyimide film having large surface roughness. There has been needed a process for producing a polyimide film having few surface flaws and reduced surface roughness.
Furthermore, the additional problem is that a laminate having high peel strength cannot be obtained when a metal layer is formed on a polyimide film by metal deposition or sputtering.
There have been various attempts to improve such lower adhesiveness of a polyimide film. For example, there has been proposed a polyimide film with improved adhesiveness, comprising 0.02 to 1% by weight of a tin, bismuth or antimony compound (Patent documents 1, 2 and 3). However, this polyimide film may have deteriorated electric properties such as electric insulation.
There has been proposed a technique for improving adhesiveness of a polyimide film by a plasma discharge treatment (Patent documents 4 and 5). However, such a discharge treatment may be inadequate to improve adhesiveness of a polyimide film, and it is lower in productivity.
Furthermore, there has been proposed a process for producing a polyimide film wherein a heat-resistant surface treatment agent (coupling agent) is applied to the surface of a polyimide film or a solidified film to give a polyimide film (such as the above-mentioned self-supporting film), and then the film is dried by heating, for the purpose of improving adhesiveness of a polyimide film (Patent documents 6-11).
However, it is pointed out that the polyimide film treated with a heat-resistant surface treatment agent may be curled, and trouble may occur during fine wiring processing in a post-processing to a long polyimide film particularly, although it has the treated surface with improved adhesiveness.
As one possible cause of curling, a polyimide film may be curled due to shrinkage of a base material during the removal of a solvent from a polyimide precursor solution, or imidization, for example. As for a multilayer film, it is known that various polyimide precursor varnishes can be flow-casted with a multilayer extruder, for example, while controlling linear expansion coefficients of the surface and of the inside of the polyimide precursor in the thickness direction, to reduce the difference in linear expansion coefficient between the convex surface and the concave surface after curling of the polyimide film, resulting in the effective prevention of curling.
However, in a monolayer film employing a single polyimide precursor varnish, it is difficult to control the curling precisely, although it is suggested that the curling is altered by varying a temperature of removing a solvent in the polyimide precursor and a heating temperature for imidization.